a) Field of the Invention
This invention relates to a novel process for the preparation of .alpha.-tocopherol useful as an antisterile vitamin, hypolipidemic, blood flow increasing agent, oxygen radial scavenger, anticytosenility agent, antioxidant and the like.
b) Description of the Related Art
.alpha.-tocopherol has heretofore been prepared by condensing trimethylhydroquinone represented by the following formula (I): ##STR1## with any one of phytols represented by the following formulae: ##STR2## in accordance with the Friedel-Crafts reaction ##STR3##
In this reaction, it is preferred to use a solvent, for example, to control the reaction heat and/or to lower the viscosity of the reaction mixture. Specifically, ethyl acetate, hexane, benzene, toluene, methylene chloride or the like is used. Among these, ethyl acetate is commonly used.
This common use of ethyl acetate as solvent for preparing .alpha.-tocopherol can be attributed to its appropriate solubility for trimethylhydroquinone (I) and also to its abundant availability at low cost in industry.
Ethyl acetate, however, is accompanied by problems such that upon washing a reaction mixture subsequent to the reaction, ethyl acetate is caused to transfer at a high rate into a water layer, its recovery rate is low, and treatment of waste water from the washing is difficult.
Further, ethyl acetate is extremely prone to hydrolysis under conditions other than neutral conditions, especially under basic conditions. It also tends to undergo a transesterification. Due to these properties, there is a further problem that when ethyl acetate is employed in the preparation of .alpha.-tocopherol, byproduction of .alpha.-tocopheryl acetate is unavoidable.
As .alpha.-tocopherol and .alpha.-tocopherol acetate have extremely close physicochemical properties, they cannot be separated and purified by any industrial purification method such as molecular distillation. Nevertheless, .alpha.-tocopherol is required to have an extremely high purity when employed as medicines, a base material for cosmetics, foods or the like. The preparation process of .alpha.-tocopherol, which makes use of ethyl acetate as a solvent, has therefore been difficult to meet the demand for such high-purity .alpha.-tocopherol.
On the other hand, use of a single kind of nonpolar solvent such as hexane, benzene or toluene tends to produce static electricity upon its charging or its transfer through piping in a dry season such as winter, whereby inflammation and/or explosion hazards are involved.
As has been described above, the conventional preparation process of .alpha.-tocopherol, which uses ethyl acetate or a single kind of nonpolar solvent, is accompanied by many problems from the viewpoints of product purity, economy, waste treatment, safety and the like, resulting in the long-standing desire for an excellent industrial solvent which can substitute for such conventional solvents.